1. Field of the Invention
The present invention relates to a projection optical system, a magnification projection optical system, a magnification projection apparatus, and an image projection apparatus.
2. Description of the Related Art
Liquid crystal projectors are widely known as image projection apparatuses. In these days, the liquid crystal projectors have employed liquid crystal panels with higher resolution, have enjoyed improved brightness due to higher efficiencies of light source lamps, and have become less expensive.
Further, small, light-weight image projection apparatuses using a DMD (Digital Micromirror Device) have been widely used not only in offices and schools but also at home. Particularly, the portability of front-type projectors has been improved so that they are also used in small meetings.
It is desired that projectors, or image projection apparatuses, be able to project a large-screen image. That is, it is desired to enlarge a projection screen. Further, it is also desired that a projection space required outside a projector be reduced as much as possible.
In order to reduce the outside projection space while enlarging the projection screen, it is desirable to incorporate the light path of an imaging light beam forming a projected image as much as possible in the projector (image projection apparatus). The following patent-related documents disclose image projection apparatuses employing such technique.
Japanese Laid-Open Patent Application No. 2002-40326 (Prior Art 1) discloses an image projection apparatus including first through fourth reflecting mirrors to obtain a wider angle of view while preventing an imaging optical system from becoming larger in size. The first reflecting mirror has a concave shape and each of the second through fourth reflecting mirrors has a convex shape. The imaging optical system is composed of the first through fourth reflecting mirrors. At least one of the first through fourth reflecting mirrors has a free-form surface so as to ensure projection performance.
Japanese Laid-Open Patent Application No. 2002-174853 (Prior Art 2) discloses an image projection apparatus that is a surface projection type display whose projection distance to a screen is reduced. The surface projection type display includes an imaging optical system composed of a pair of a concave mirror and a convex mirror having divergence action and a projection lens.
Japanese Examined Patent Application Publication No. 6-91641 (Prior Art 3) discloses an image projection apparatus that is a video projector. The first mirror surface of the imaging optical system of the video projector has a convex shape so as to reduce the thickness of the video projector.
The image projection methods of Prior Art 1 and 3 perform image formation using only one or more reflecting mirrors to magnify and project a single lightbulb image on a screen. Therefore, these image projection methods enjoy the merit of no generation of chromatic aberration in principle. In the case of displaying images of red, green, and blue separately, using three lightbulbs instead of a single lightbulb, and combining the separate images on a screen, the intervention of a color combining part such as a cross prism or a Philips prism is necessary, thus resulting in the generation of chromatic aberration at the time of combining colors. However, the imaging optical system composed of only reflecting surfaces cannot correct chromatic aberration.
According to an image projection apparatus disclosed in Japanese Laid-Open Patent Application No. 2001-264627 (Prior Art 4), each light beam from an image display panel is guided to a screen successively by the positive-power imaging lens system and the reflection optical system including a negative-power curved surface mirror of a magnification projection optical system to be focused onto the screen.
The screen and the imaging lens system are set so that the height of the screen is offset from the height of the imaging lens system. The light beams are reflected back from the mirror to be guided to the screen. Therefore, the optical path length of the imaging light beam differs between the upper side and the lower side of the center part of the magnified projected image on the screen (corresponding to the center part of the image display panel), thus resulting in the generation of a so-called “trapezoidal distortion.”
The trapezoidal distortion is correctable by “keystone correction,” which, however, is likely to degrade the quality of the magnified image on the screen.
It is well known to provide a convex mirror between the imaging lens system and the screen so that the convex mirror is decentered with respect to the optical axis of the imaging lens system in order to reduce the trapezoidal distortion. In the case of providing the convex mirror with decentration, the convex mirror is provided on the imaging lens side of the screen-side focus position of the imaging lens system so as to extend the focus position of a projection lens by the negative refracting power of the convex mirror.
In order to realize a low-profile, large-screen magnification projection apparatus with the above-described configuration, the negative power of the convex mirror may be increased to widen an angle of view. This, however, requires stricter shape accuracy and stricter assembly tolerance of the convex mirror, and also increases distortion.
Distortion may be reduced by decreasing the refractive power of the convex mirror by increasing the distance between the imaging lens system and the convex mirror. As the distance between the imaging lens system and the convex mirror increases, however, the convex mirror becomes larger in size to increase its cost. Further, the magnification projection apparatus is also likely to become larger in size.
According to Japanese Laid-Open Patent Application No. 2002-296503, a magnification projection optical system is composed of only reflecting mirrors. In the case of thus obtaining desired optical performance without using a lens optical system, it is necessary to set the surface accuracy and position accuracy of each reflecting surface to extremely high values, thus requiring stricter assembly accuracy of the magnification projection optical system.